Exhaust gas recirculation in a two stroke engine

ABSTRACT

A method of operating a two stroke cycle crankcase scavenged internal combustion engine, comprising selectively delivering exhaust gas from a location downstream of the engine exhaust port to the engine crankcase to be delivered together with air in the crankcase to the engine combustion chamber. The quantity of exhaust gas delivered to the crankcase during each engine cycle is controlled according with engine operating conditions and the rate of supply of exhaust gas to the crankcase is controlled relative to engine operating conditions such as by an ECU managed valve that responds to engine operating conditions such as load, speed and temperature. The exhaust gas is admitted to the crankcase during a period when the pressure in the crankcase is below that of the available exhaust gas.

This invention relates to internal combustion engines operating on thetwo stroke cycle and to the management of the combustion process thereofto control the level of contaminants in the exhaust emissions.

It has in the past been recognised that two stroke cycle engines exhibitpoor performance in the area of fuel consumption and also in the area ofthe level of harmful emissions in the engine exhaust. However, there aresubstantial benefits to be obtained by wider use of engines operating onthe two stroke cycle. Firstly, because of their relatively simpleconstruction, and secondly, because of their relatively small physicalsize and resultant high power to weight ratio. There has accordinglybeen considerable development in recent years directed to the control ofthe combustion process of two stroke cycle engines in a manner to reducethe level of emissions in the exhaust, and/or reduce the fuelconsumption.

It has been recognised that the introduction of exhaust gas into thefuel/air mixture prior to the ignition thereof can contribute to areduction in the production of NO_(x) (oxides of nitrogen) during thecombustion process, as the presence of exhaust gas in the fuel/airmixture reduces the resultant temperature and pressure in the enginecylinder resulting from the combustion, which is contrary to the hightemperature and pressure conditions that promote the creation of NO_(x).This process of mixing exhaust gas with the fuel/air mixture is commonlyreferred to as exhaust gas recirculation (EGR) and is typically achievedby bleeding a controlled quantity of exhaust gas from the exhaust systeminto the air induction manifold of the engine.

Although this procedure has been used successfully in four stroke cycleengines, it is not as effective when applied to two stroke cycleengines, due principally to the low level of vacuum existing downstreamfrom the conventional throttle in the air induction system. Hence, thiswould result in a low rate of intake of exhaust gas and in the case of amulti cylinder engine, a poor distribution of the exhaust gas in theinduced air. Further, particularly in the case of a crankcase scavenegedengine, there is a significant time lag in the response by the engine tothe introduction of the exhaust gas to the induction system due to thedistance it is required to travel before entering the combustionchamber. Also, where exhaust gas is introduced into the inducion systemof a crankcase scavenged two stroke cycle engine as the air capacity ofthe crankcase is greater than that of the cylinder, there is a furthertime lag in a change in rate of exhaust gas supply to the crankcasebeing seen in the engine cylinder. Also, the particulate materialsnormally present in exhaust gas can build up on mechanisms in theinduction system of such two stroke cycle engines such as throttlevalves and crankcase reed valves, and hence interfere in the effectiveoperation thereof.

It has been previously proposed in publications such as InternationalPatent Publication WO 79/00757 by J. P. Soubis and U.S. Pat. No.3,581,719 by Gau to recycle exhaust gas into the combustion chamber forthe purpose of returning thereto any unburnt fuel that may be in theexhaust gas. It has long been recognised that carburetted two strokecycle engines exhibit the problem that part of the air and fuel chargethat enters the combustion chamber passes out through the exhaust portprior to the commencement of combustion. The above referred to priorpatents each are directed to overcoming this problem by re-directing thefuel rich portion of the exhaust gas back into the air intake orcrankcase for recycling into the combustion chamber. As the recycledportion of the exhaust gas is primarily fuel and fresh air, it will nothave a significant effect in the control of the generation of NO_(x),the problem the present invention is directed to overcoming.

It is proposed in U.S. Pat. No. 4,213,431 by Onishi to recycle exhaustgas into the air induction system of a two stroke cycle engine with theintent of control of the generation of NO_(x) during the combustionprocess. In this proposal the exhaust gas is introduced directly intothe air induction passage upstream of the carburettor. A valve isprovided in the duct conveying the exhaust gas to the induction passage,the valve being temperature activated.

It is the object of the present invention to provide a method forintroducing exhaust gas into the combustion chamber of a two strokecycle engine whereby effective control of the amount and distribution ofthe exhaust gas can provide the most beneficial results in themanagement of exhaust emissions.

With this object in view, there is provided a method of controlling thegeneration of NO_(x) in a fuel injected two stroke cycle crankcasescavenged internal combustion engine comprising selectively deliveringexhaust gas from a location downstream of the engine exhaust port to theengine crankcase to be delivered from the crankcase to a combustionchamber of the engine, controlling the quantity of exhaust gas deliveredto the crankcase during each cycle of the engine in accordance with theengine operating conditions, and timing the delivery of the exhaust gasto the crankcase with respect to the cycle of the engine.

As the admission of the exhaust gas to the crankcase is principally forthe control of exhaust emissions, it is not necessary, and in somecircumstances can be undesirable, to admit exhaust gas to the crankcaseunder all operating conditions. Accordingly, it is desirable to beselective in the introduction of the exhaust gas to the crankcase andalso to control the rate of supply of exhaust gas. This control of theexhaust gas supply to the engine crankcase can be achieved by an ECUmanaged control valve provided between the exhaust system of the engineand the engine crankcase to control the exhaust gas flow to thecrankcase. The ECU managing the control valve preferably receives inputsregarding engine operating conditions and in particular, engine load,speed and temperature conditions. The ECU determines from these inputswhen exhaust gas is required to be introduced to the crankcase and thequantity thereof required.

Preferably, the control valve incorporates a position feedback means toindicate to the ECU the actual position of the valve to thereby permitcomparison of the actual position with the required position, therebyenhancing the accuracy of the control of the rate of supply of exhaustgas to the crankcase. Alternatively, the ECU can be programmed todetermine the actual mass of exhaust gas delivered to the crankcase eachcycle and to compare that mass with the required mass of exhaust gas forthe existing engine operating conditions. Any correction required canthen be effected by adjustment of the rate of supply of exhaust gas tothe crankcase via the ECU controlled valve.

In a multi-cylinder two stroke cycle crankcase scavenged engine, whereeach cylinder has an individual crankcase compartment, a plenum chambermay be provided which communicates individually with the crankcase ofeach cylinder, with exhaust gas from one or more of the engine cylindersbeing provided to the plenum chamber. Under normal conditions, it mayonly be necessary to supply exhaust gas from one or some of thecylinders to the plenum chamber, even where a greater number ofcylinders are supplied with exhaust gas from the plenum chamber.

In part, the exhaust gas performs the emission control function byreducing the overall cycle temperature and pressure in the enginecylinder during combustion, as the exhaust gas has a higher specificheat than air and hence will reduce the overall cycle temperature of thegases in the combustion chamber. As high temperature is one of therequirements for the production of NO_(x), this reduction in overallcycle temperature contributes to a reduction in the production ofNO_(x). It is also to be noted that the exhaust gas of a two strokecycle engine has a higher oxygen content than that of a four strokecycle engine and therefore more exhaust gas is required to be recycledin a two stroke engine to receive a comparable level of NO_(x) control.

The use of a plenum chamber in a multi-cylinder engine can readily bemade to contribute to a reduction in the temperature of the exhaust gasdelivered to the combustion chamber and in addition, provision can bemade to enhance the dissipation of heat from the plenum chamber toachieve a further temperature reduction. Also a heat exchanger can beincorporated in the path of the exhaust gas to the engine crankcase tofurther contribute to a reduction of the temperature of the exhaust gasprior to admission to the crankcase. Conveniently, the duct conveyingthe exhaust gas to the cylinder or cylinders or to the plenum chambercan be cooled by external fins or by liquid or water cooling such asfrom the engine cooling system.

In accordance with another aspect of the invention, there is provided amethod as claimed in any one of claims 1 to 7 including cooling theexhaust gas prior to delivery to the crankcase.

In one embodiment, a piston controlled port can be provided in the lowerportion of the engine cylinder wall to communicate with the crankcase ofthat particular cylinder and also with the supply of exhaust gas, suchas via the plenum chamber. Preferably, theh port communicates with theexhaust system via an ECU controlled valve as previously described. Theport is located so that it will be exposed whilst the piston is within alimited extent of movement on either side of the top dead centreposition thereof which represents the period of minimum pressure(sub-atmospheric) in the crankcase. Conveniently, the port can belocated so as to be open for approximately 40° to 60° of crankshaftrotation to each side of the top dead centre position of the pistonstroke.

It may be convenient in some engine configurations to provide two ormore such piston controlled ports for each crankcase to provide arelatively large flow area for the exhaust gas into the crankcase.Preferably, the port or ports have the major dimension thereof in thecircumferential direction of the cylinder to provide the maximum openport area during the restricted port open period. It has been found thatthe use of a piston controlled port to control the timing of admissionof the exhaust gas results in improved equitable cylinder to cylinderdistribution of the exhaust gas.

Preferably, the ECU managing the degree of opening of the valve isprogrammed to control the valve by reference to a speed/load basedlook-up o map. The load may be plotted on a FPC (fuel per cycle) basis.In addition, the ECU preferably also responds to engine temperaturesince, for example, at some cold start conditions, the addition ofexhaust gas can be detrimental to the engine operating stability.

In an engine where catalyst unit is provided in the exhaust system theexhaust gas can be taken from the exhaust system either upstream ordownstream of the catalyst unit.

The invention will be more fully understood from the followingdescription of two alternative arrangements of crankcase scavenged twostroke cycle engines.

In the drawings:

FIG. 1 is a transverse sectional view of a crankcase scavenged twostroke cycle engine incorporating one embodiment of an exhaust gasrecirculation system; and

FIG. 2 is a longitudinal sectional plan view of a portion of a threecylinder engine incorporating the exhaust gas recirculation system asshown in FIG. 1.

Referring now to FIG. 1 of the drawings, there is shown a partialtransverse section of a crankcase scavenged two stroke cycle enginewherein the cylinder block 25 is in cross section and the cylinder head26 and fuel injector equipment 27 are in full outline. For the purposesof clarity, the piston and connecting rod are not shown. The engine isbasically of conventional construction having a cylinder 30 in which apiston (not shown) reciprocates and is connected by a connecting rod(not shown) to a crankshaft shown diagrammatically at 31. The cylinder30 has an exhaust port 32 and a plurality of transfer ports, two ofwhich are shown at 33 and 34 to provide communication between acrankcase 35 and the cylinder 30, subject to the position of the pistonwithin the cylinder 30 as per the conventional two stroke cycleprinciple. FIG. 1 may be of a single cylinder engine or in-linemulti-cylinder engine as shown in FIG. 2.

The exhaust port 32 communicates with an exhaust passage 36 which inturn communicates with an exhaust pipe 37 in the mouth of which arelocated conventional exhaust catalyst elements 38. Downstream of thecatalyst elements 38, the exhaust pipe 37 communicates with a branchpassage 39 which leads to a cavity 40 in the cylinder block in FIG. 1,the branch passage 39 communicated with the exhaust pipe 37 downstreamof the exhaust catalyst elements 38. However, it may alternativelycommunicate with the exhaust pipe 37 upstream of the catalyst elements38 where the pressure of the exhaust gas is higher, the temperature islower, and the oxygen content is lower.

Communication between the passage 39 and the cavity 40 is under thecontrol of the valve 41 being part of a solenoid valve mechanism 42. Thecavity 40 communicates with an internal plenum chamber 43 provided inthe cylinder block which, in a multi cylinder engine as shown in FIG. 2,communicates individually with each cylinder of the engine through arespective exhaust gas recirculation port 45. The ports 45 arepositioned in the wall of the respective cylinders 30 so that during aselected portion of each cylinder cycle, the respective ports 45 areuncovered by the pistons to permit exhaust gas to flow from the plenumchamber 43 into the respective crankcase 35 of each cylinder 30 of theengine.

As previously indicated, the preferred timing of the opening and closingof the port 45 is within 40° to 60° before and after the top dead centreposition of the piston in the respective cylinder 30 and preferablybetween 45° and 55°. The timing of the supply of exhaust gas to thecrankcase is preferably at a period in the engine cycle when thepressure in the crankcase is below that in the exhaust system. It willbe appreciated that the communication between the crankcase 35 and theEGR port 45 is determined by the location of an appropriate aperture inthe crankcase wall with respect to the path of the lower edge of theskirt of the piston as is common technology in relation to the controlof the flow of gases through the transfer ports of two stroke cycleengines, such as the transfer ports 33 and 34 as shown in FIG. 1.Further, it is preferred that the port 45 is located on the side of thecylinder against which the piston is thrust so as to effectively sealthe pod 45 when required.

As referred to previously, it is desirable to control the delivery ofexhaust gas through the port 45 in accordance with variations in engineoperating conditions and for this purpose, the solenoid mechanism 42, orother suitable mechanism that controls the operation of the valve 41 isunder the control of a programmed electronic control unit (ECU) 46incorporating appropriate look-up maps. Usually, the map is arranged sothat the valve 41 controlling the exhaust gas flow to the crankcase 35is ramped rapidly from closed to open once the fuelling rate increasesabove a selected level. The solenoid mechanism 42 is typically providedwith a valve element position sensor 47 which provides feedbackinformation to the ECU 46 to facilitate the accurate control of theposition of the valve 41 to achieve the required rate of supply ofexhaust gas to the respective crankcase cavities 35 of the engine. TheECU 46 typically receives inputs indicating the engine speed, engineload, and engine temperature. Provision can also be made for the ECU tomonitor and adjust the movement of the valve 41 to detect and correctfor the effects of carbon build-up on the valve of seat in which itoperates.

As an alternative to the use of look-up maps to control the rate ofsupply of exhaust gas, the ECU can be programmed to determine the massof air entering each crankcase 35 and to determine the combined exhaustgas and air mass in the respective crankcase 35 at a selected point inthe cycle of that engine cylinder 30. The mass of air entering thecrankcase 35 can be determined by a conventional hot wire air flow meterin the air induction passage, and the mass of air and exhaust gas can bedetermined by measuring the temperature and pressure in the crankcase 35at a preset point in the engine cycle when the volume of the space inthe crankcase 35 is known. From these two mass determinations, theactual mass of exhaust gas can be determined and compared with therequired amount of exhaust gas, thus determining if adjustment isrequired to be made to the rate of supply of the exhaust gas. Thismethod of determining the exhaust gas content within the crankcase canbe used in conjunction with other forms of control of EGR than thatdescribed herein. Also, by determining the air mass in the crankcasewhen no exhaust gas is present therein, the accuracy of the measurementprocess can be checked by comparing the calculated air mass with the airmass as determined by the hot wire air flow meter.

Other engine operating parameters that can be controlled, in conjunctionwith the supply of exhaust gas to the crankcase 35, include advance ofthe ignition spark to increase combustion temperature, and use of a backpressure valve in the exhaust system, downstream of the point of exhaustgas take-off to control the rate of exhaust gas available for supply tothe combustion chamber. The greater the back pressure in the exhaustsystem, the greater the pressure and hence the rate of supply of exhaustgas for admission to the crankcase 35.

Also, the exhaust gas to be delivered to the engine cylinder 30 can bepassed through a heat exchanger or otherwise cooled means prior to entryto the crankcase 35 in order to increase the density thereof whereby agreater mass of exhaust gas would then be available for delivery to thecrankcase 35. In FIG. 1, the branch passage 39 communicates with theexhaust pipe 37 downstream of the catalyst element 38. However, it mayalternatively communicate with the exhaust pipe 37 upstream of thecatalyst element where the pressure of the exhaust gas is higher and thetemperature lower.

In practice, it has been found convenient to provide multiple deliverylocations for the exhaust gas to the plenum chamber to assist inachieving substantial uniform distribution of the exhaust gas from acommon plenum chamber into the respective crankcase of each cylinder.

Also, in the higher range of operating speeds, the rate of rise of thecrankcase pressure, after top dead centre position of the piston, issuch that a reverse flow of air from each respective crankcase throughthe ports 45 can occur. This can lead to a dilution of the exhaust gasin the plenum chamber and possibility of unequal distribution of exhaustgas to the respective engine cylinders. This problem can be at leastreduced by suitable selection of the length of the ports 45 between theplenum chamber 43 and the crankcase chambers 35, so that any reverseflow of gas from the crankcase is substantially retained with the port45 associated with that crankcase chamber 35 and not passed into theplenum chamber 43.

The claims defining the invention are as follows:
 1. A crankcasesscavenged two stroke cycle engine, comprising means to selectivelyconvey exhaust gas from an engine exhaust system to at least onecrankcase of the engine, means to regulate the mass of exhaust gasdelivered to said crankcase each engine cycle, and means for controllingthe timing of the delivery of said exhaust gas to the crankcase inrelation to the engine cycle;wherein the means to regulate the mass ofexhaust gas includes valve means selectively operably to control tocontrol exhaust gas flow along said means to convey the exhaust gas; andwherein said means to regulate the mass of gas flow includes feed-backmeans to determine the actual mass of exhaust gas delivered to thecrankcase per cycle and means to correct variations between the requiredquantity and delivered quantity of exhaust gas delivered per cycle.
 2. Amethod of controlling the generation NO_(x) in a fuel injected twostroke cycle crankcase scavenged internal combustion engine comprisingselectively delivering exhaust gas from a location downstream of anexhaust port of the engine to a crankcase of the engine to be deliveredfrom the crankcase to a combustion chamber of the engine, controlling,by an electronic control unit, a quantity of exhaust gas delivered tothe crankcase during each cycle of the engine in accordance with engineoperating conditions, and timing the exhaust gas delivery to thecrankcase with respect to engine cycle.
 3. A method as claimed in claim2 wherein the control of the quantity of exhaust gas delivered to thecrankcase is determined with reference to at least one of engine load orspeed.
 4. A method as claimed in claim 2 wherein the delivery of theexhaust gas to the crankcase commences between 55 to 45 degrees before atop dead center point in a combustion chamber cycle.
 5. A method asclaimed in claim 4 wherein the control of the quantity of exhaust gasdelivered to the crankcase is determined with reference to at least oneof engine load or speed.
 6. A method as claimed in claim 2 wherein thedelivery of the exhaust gas to the crankcase commences between 60 and 40degrees before a top dead center point in a combustion chamber cycle. 7.A method as claimed in claim 6 wherein the control of the quantity ofexhaust gas delivered to the crankcase is determined with reference toat least one of engine load or speed.
 8. A method as claimed in claim 2wherein the timing of the delivery of the exhaust gas to the crankcaseis controlled by a location of a port in the crankcase that is openedand closed in response to movement of a piston in a cylinder.
 9. Amethod as claimed in claim 8 wherein the control of the quantity ofexhaust gas delivered to the crankcase is determined with reference toat least one of engine load or speed.
 10. A method as claimed in claim 8wherein the delivery of the exhaust gas to the crankcase commencesbetween 60 and 40 degrees before a top dead center point in a combustionchamber cycle.
 11. A method as claimed in claim 10 wherein the controlof the quantity of exhaust gas delivered to the crankcase is determinedwith reference to at least one of engine load or speed.
 12. A method asclaimed in claim 8 wherein the delivery of the exhaust gas to thecrankcase commences between 55 to 45 degrees before a top dead centerpoint in a combustion chamber cycle.
 13. A method as claimed in claim 12wherein the control of the quantity of exhaust gas delivered to thecrankcase is determined with reference to at least one of engine load orspeed.
 14. A method as claimed in claim 2 wherein the delivery of theexhaust gas to the crankcase is commenced after a closing of transferports that communicate the crankcase with the combustion chamber.
 15. Amethod as claimed in claim 14 wherein the control of the quantity ofexhaust gas delivered to the crankcase is determined with reference toat least one of engine load or speed.
 16. A method as claimed in claim14 wherein the delivery of the exhaust gas to the crankcase commencesbetween 55 to 45 degrees before a top dead center point in a combustionchamber cycle.
 17. A method as claimed in claim 16 wherein the controlof the quantity of exhaust gas delivered to the crankcase is determinedwith reference to at least one of engine load or speed.
 18. A method asclaimed in claim 14 wherein the timing of the delivery of the exhaustgas to the crankcase is controlled by a location of a port in thecrankcase that is opened and closed in response to movement of a pistonin a cylinder.
 19. A method as claimed in claim 18 wherein the controlof the quantity of exhaust gas delivered to the crankcase is determinedwith reference to at least one of engine load or speed.
 20. A method asclaimed in claim 14 wherein the delivery of the exhaust gas to thecrankcase commences between 60 and 40 degrees before a top dead centerpoint in a combustion chamber cycle.
 21. A method as claimed in claim 20wherein the control of the quantity of exhaust gas delivered to thecrankcase is determined with reference to at least one of engine load orspeed.
 22. A method as claimed in claims 2, 14, 8, 18, 6, 20, 10, 4, 16,12, 3, 15, 9, 19, 7, 21, 11, 5, 17, or 13 including determining anactual mass of exhaust gas delivered to the crankcase per cycle andcorrecting variations between a required quantity and delivered quantityof exhaust gas delivered to the crankcase per cycle.
 23. A method asclaimed in claims 2, 14, 8, 18, 6, 20, 10, 4, 16, 12, 3, 15, 9, 19, 7,21, 11, 5, 17 or 13 including cooling the exhaust gas prior to deliveryto the crankcase.
 24. A method as claimed in claim 23 includingdetermining an actual mass of exhaust gas delivered to the crankcase percycle and correcting variations between a required quantity anddelivered quantity of exhaust gas delivered to the crankcase per cycle.25. A method as claimed in claims 3, 15, 9, 19, 7, 21, 11, 5, 17, or 13wherein the control of the quantity of exhaust gas delivered to thecrankcase is determined with reference to engine temperature.
 26. Amethod as claimed in claim 25 including cooling the exhaust gas prior todelivery to the crankcase.
 27. A method as claimed in claim 26 includingdetermining an actual mass of exhaust gas delivered to the crankcase percycle and correcting variations between a required quantity anddelivered quantity of exhaust gas delivered to the crankcase per cycle.